Thermionic amplifier



Aug. 29, 1933. c ow 1,924,268

THERMIONIG AMPLIFIER Filed Oct. 23, 1929 Patented Aug. 29, 1933 UNITED I s -A TES P V a TnEnMIoNIo MPLIFIER 'CecilOswald. Browne, Ealing Common, London, v England, assignor, by mesne 'assignments, to Radio Corporation'of America, New York, N. Y., a Corporation of Delaware Application combats, 1929, Serial No. 401,930, and in Great Britain November 17, 1928 5 Claims. (Cl. 179-471 The present invention relates to thermionic amplifiers and has as its object the provision of an amplifier of small size, great simplicity and high magnification at acoustic or audio frequencies. I

According to the present invention an audio frequency thermionic amplifier, for example for use in recording sound or in'the reproduction from a sound record, comprises a four electrode valve immediately followed by a five electrode valve or pentode.

The invention is illustrated by way of example in the accompanying drawing. a

In the drawing the first valve V1 is a four electrode valve commonly known as a screened grid valve. .The inner grid 1, or that nearer the filament 2, is connected to one input terminal 3 through a condenser 4, the other input terminal p 7 5 being connected to the filaments 2, 6 of both a circuit of the screen grid valve V1, the two cir- "'cuits may be connected with the same source valves V1 and V2. The usual grid leak 7 is provided and the inner grid 1 may, through the leak, be given a negative bias of about 2 volts with respect to the filament.

tive with respect to the filament 2. The anode 9 of the valve V1 is connected throughan ironcore choke 10 with a suitable potentialof substantially 150 volts. The anode 9 is connected through a condenser 11 with the inner or con-. trol grid 12, of a live element valve V2 of the type known as a pentode, a grid leak 13 being through the primary winding of an output.

transformer 17 to a suitable current source of about 150 volts. It will be noted that since this is the same potential as required for the anode of supply, as indicated in the drawing. This is a relatively low voltage and is permissible with the amplifier arrangement shown in the drawing, comprising a pentode or stabilized low voltage power output valve, impedance coupled with a high gain voltage amplifier employing a high impedance stabilized valve such as a screen grid valve. This amplifier arrangement is therefore relatively simple and effective both in gain and quality and is economical of voltage,

The outergrid 8 may be given a potential of about 75 volts posi- As characteristic of the pensince, both valves are adapted to be connected to the same. low. voltage, supply source. The secondary winding of the transformerl'l may be. connected. with any suitable output device or circuit, notshown. 1

a .The amplifier illustrated is adapted for use.

in reproducing sound from a photographic soundjrecordthe terminals 3, 5, being adapted to receive the electrical input therefrom. For this purposethe resistance 18fand the potential divider 19 are preferably provided in known manner.

It should be noted, asobvious to onev Skilled in theart, that an amplifier of the character above described, employing .two high gain" valves, not only providesfor a relatively high amplification of the input signals and hence mayhave a relatively high sensitivity with a relatively low current drain upon the anode po--.

'tential supplysource, but also has a desirable frequency response characteristic whereby it is particularly well adapted for use with known sound producingdevices such as loud speakers for example.v This is for the reason that, as is well known, loud speakers and the like are more sensitive in the higher frequency end of the audio frequency range and therefore tend to provide a greater response in the higher frequency range. The pentode type of tube, however, tends to reduce the high frequency response because of the shunt capacity imposed "by it across the circuit, between its input elec-- trode and cathode, thereby tending to compen-I I sate for the response characteristic of a device connected in its output circuit. Bothdevices being of the screen grid or stabilized type also tends to improve the transmission characteristic of the amplifier by eliminating feed-back through the inter-electrode capacity of the devices. 1 I

The amplifier may however, be used in conjunction with a wireless receiver a detector be-.

circuit of the valve V1.

, I claim:

l. The combination, in an audio frequency amplifier, 'of a screen grid high impedance input valve, impedance coupling means providing an input circuit therefor, a second impedance, coupling means providing an output circuit thereing connected or coupled to. the control grid for, a power output valve of the pentode type I connected with said output circuit whereby said valves areimpedance coupled, and adapted to operate in connection with a common low voltage source of anode current, and means adapted the screen grid typewh'ereby it is adapted to to provide a common low voltage source of operating potentials for said valves at values providing'relatively low current consumption and' relatively high power output in response to signal potentials of'relatively low amplitude.

,2. man audio frequency amplifier having input and. output terminals, means providing a relatively high signal gain from said input terminals to said output terminals and a frequency compensation characteristic, said means including a first stage high impedance screen grid valve having a relatively high amplification constant connected with said input terminals, I a second stage valve having a relatively high amplificationconstant connected with said' output terminals, and impedance coupling means interposed between said valves whereby they are connected in cascade relation to each other in said amplifier, said-second named valve being of the,

low anode voltage, pentode type, and having input electrodes arranged to provide a highfrequencyattenuating. capacity in shunt to a portion o'f the amplifier circuit. 7 I I 3. In an audio frequency amplifier, the combination of a pair of valves each having a rela-' tively high amplification constant, and impedance" coupling means interposed between iand connecting them in cascade relation, whereby said valves provide a first and aisecond stage of said amplifier, saidfirst stage valve being of be connected with said impedance coupling means and operated in response to low potential signals, and said second stagevalve being of the low voltage, pentode type; whereby it iniparts a frequency"attenuating;characteristic to impulses ,to said output valve, said amplifier be the audio frequency amplifier in a higher frequency portion of the audio frequency range.

4. In an audio frequency amplifier, the combination with a low voltage power output valve of the pentodetype adapted for relatively high signal amplification, and power output, at low operating and signal potentials, of means for driving said valve in responserto relatively low signal potentials and with relatively low operating potentials and current, including a screen grid high impedance valve adapted to receive signal impulses, and impedance coupling means conlnected'between' said last named valve and the output valve for transmitting amplified signal 7 ing thereby stabilizedandsensitive to signals of relatively low amplitude.

5. The combination, in an audio frequency amplifier; of a screen grid high impedance input. valve, i ipedance coupling means providing an input coupling network therefor, a second impedance couplingmeans providing an output circuit therefor, a power output valve of the pentode' type connected with said output circuit whereby said valves are impedance coupled and 00' oscni OsWALD BROWNE. 

